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I have upgraded my bike with a new front suspension fork. Bought and fitted an adaptor from Shimano as the fork is laid out for 160mm (Postmount 6) and my disc front wheel brake rotor is 180mm. However, when I fitted the wheel I noticed that the outside diameter of the rotor catches against the caliper.

The problem disappears if I fit a spacer between the fork and the adaptor but that is not a solution that I am keen on. The wheel can rotate but the problem is clearly noticeable.

The fork and the caliper are both post mount and I have double checked that the adaptor is fitted the right way up.

What am I missing?

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    I have had this problem in the past. I ended up using a 1/2mm spacer between the caliper and adapter. What caliper do you have?
    – mattnz
    Jan 30 '21 at 19:52
  • Good to know that I am not the only one:-) The calipers are Shimano Deore. The current equivalent would be the MT500.
    – Fredrik
    Jan 30 '21 at 19:59
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    Rather than a spacer, you might try some metal shim between the two flat faces of the fork and the adapter. Probably better than using washers which aren't as wide.
    – Criggie
    Jan 30 '21 at 21:51
  • Thanks for that suggestion. You are right that shims would be a better option. I will get to work on it.
    – Fredrik
    Jan 31 '21 at 12:14
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You're not missing anything. Sometimes this just happens.

There is a bigger question at hand regarding what aspects of the relevant parts dimensions are truly standardized, and which are the kind of informal follow-the-leader standards the bike industry often uses. It doesn't really matter and I couldn't write that answer.

Use up to 2mm or so of shim/washer on each bolt. Normal M6 washers are fine, or you could use fancy ground ones if you have them. Make sure the bolts keep at least 6mm of thread engagement and use longer bolts if not. Other than that, all you care about is the pad contact on the rotor. You are shooting for no overhang of the pad, such that the entire pad face is contacting with no potential for a lip to build up as the pad wears.

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